AnimatedSwitcherTransitionBuilder typedef Null safety
Signature for builders used to generate custom transitions for AnimatedSwitcher.
child should be transitioning in when the
animation is running in
the forward direction.
The function should return a widget which wraps the given
child. It may
also use the
animation to inform its transition. It must not return null.
typedef AnimatedSwitcherTransitionBuilder = Widget Function(Widget child, Animation<double> animation);